1. Field of the Invention
The invention relates to a manufacturing method for an oxygen sensor that has an oxygen sensor element coated with a Pt film and, more particularly, to a manufacturing method for an oxygen sensor having excellent detection accuracy.
2. Description of Related Art
In an existing art, an oxygen sensor (O2 sensor) has an oxygen sensor element, and the oxygen sensor element is accommodated in a housing. For example, in the case of a closed-end cylindrical (cup-shaped) oxygen sensor element, there is known an oxygen sensor element that includes a cup-shaped solid electrolyte element having oxygen ion conductivity, an inner electrode that is provided on the inner surface of the solid electrolyte element and an outer electrode that is provided on the outer surface of the solid electrolyte element.
In such an oxygen sensor element, the inner electrode is brought into contact with the atmosphere to serve as a reference electrode and the outer electrode is brought into contact with a measured gas to serve as a measuring electrode. By so doing, the concentration of oxygen in exhaust gas from an internal combustion engine is measured.
In recent years, in order to improve the performance of an oxygen sensor element, various studies have been made on the outer electrode. For example, in order to improve low-temperature operability and gas responsiveness, an oxygen sensor element of which the mean particle diameter of electrode material crystals of an outer electrode and the thickness of the outer electrode are improved is disclosed (for example, see Japanese Patent Publication No. 8-20404). Here, a solid electrolyte element that constitutes the oxygen sensor element is made of stabilized zirconia, and the surface of the solid electrolyte element is coated with a platinum (Pt) film as an electrode material.
A coating method for such a Pt film may be generally, for example, a coating method using PVD, CVD, or the like. For example, there are suggested a method in which a Pt film is coated by sputtering in an atmosphere of nitrogen (for example, see Japanese Patent No. 3094382) and a method in which the (100) plane of an Si substrate is subjected to etching to roughen the surface, an yttria-stabilized zirconia layer is laminated on the surface and the yttria-stabilized zirconia layer is coated with a Pt film in an atmosphere of Ar gas (for example, see Japanese Patent Application Publication No. 2000-277818 (JP 2000-277818 A).
Incidentally, in air-fuel ratio control over an internal combustion engine of an automobile, the air-fuel ratio is adjusted on the basis of an output voltage of an oxygen sensor element, and the oxygen sensor element has such a characteristic that the output voltage steeply varies around a stoichiometric air-fuel ratio.
Therefore, by adjusting the air-fuel ratio (executing feedback control) on the basis of the output voltage of the oxygen sensor element using the characteristic that the output voltage steeply varies, the internal combustion engine is allowed to operate at substantially an ideal air-fuel ratio. Note that, generally, in an oxygen sensor element, (air-fuel ratio at the time when the output voltage steeply varies)/(stoichiometric air-fuel ratio) is denoted by a control value λ and then air-fuel ratio control is executed on the basis of the control value λ.
The output voltage of the oxygen sensor element desirably varies quickly in response to a change between rich/lean atmospheres irrespective of the concentration of gas; however, the above described existing oxygen sensor elements do not sufficiently have such trackability (responsiveness).
The reason will be described below. As described above, the oxygen sensor indicates a change of a measured gas atmosphere as an output value with reference to the amount of oxygen in the atmosphere. Therefore, the trackability of the output voltage depends on the ability to quickly change an oxygen condition of a solid electrolyte interface via the measured gas-side electrode. Here, in a high-concentration gas environment, an oxygen condition quickly varies, so the above described responsiveness does not significantly matter. However, in an environment of which the concentration of a measured gas is low, the response time of the oxygen sensor element tends to extend with a variation in the oxygen condition. Therefore, there may temporarily occur a deviation between the oxygen concentration of the measured gas and the output voltage of the oxygen sensor. Thus, controllability of air-fuel ratio control deteriorates and, as a result, emissions may deteriorate.
On the other hand, in the above described oxygen sensor elements, the control value λ after usage for a certain period may vary from the control value λ immediately after production. As the control value λ varies, it may lead to a situation that the initially set exhaust gas purification performance of a catalyst device becomes insufficient thereafter.
The reason will be described below. In comparison with an initially set condition of an oxygen sensor, the condition of the oxygen sensor that has been endured in a vehicle is such that the forms of the electrode and electrode interface that are gas reaction portions vary because they are subjected to various thermal history and exhaust gas environments. As a result, the responsiveness, amount, diffusibility, and the like, of gas fluctuate, so the output of the oxygen sensor becomes unstable (the control value λ varies).